1. Field of the Invention
This invention relates to digital image signal processing and, in particular, to digital imaging structures and processes using an analog buffer memory.
2. Description of Related Art
Digital image processing can be used to convert an optical image to electronic image data, which is then converted to digital data for display or storage on a recording medium, such as a memory card. The stored digital data can later be retrieved to reproduce the image on a display device, such as a computer monitor, or on a hard copy medium, such as a color printer or photographic film.
FIG. 1 is a block diagram of a digital image processing system, in particular, a digital still camera (DSC) 100. DSC 100 includes an image sensor 110, such as a CMOS sensor or a charge-coupled device (CCD), pre-processing circuitry 115, an analog-to-digital (A/D) converter 120, an image processor and compressor 130, a digital memory device 140, read and interface circuitry 150, a liquid crystal display (LCD) 155, and external digital displays, such as a color LCD (not shown) or an external digital memory card 160. Generally, the image of an object is electrically processed by image sensor 110, which converts reflected light from the object into electrical signals, such as voltages.
The analog-to-digital (A/D) converter 120 then samples the analog signal and converts the signal into digital data. Some advantages of digitized information include processing flexibility and higher signal to noise ratio during transmission to reduce error rates and improve picture quality. However, digital data can require very large memory storage capacities and transmission bandwidths.
For example, typical VGA images have a spatial resolution of approximately 307K pixels/frame (i.e., 480 rows with 640 samples or pixels per row). In a digital image, the image is captured, processed, stored, and transmitted as an array of numerical values. The image is divided up into squares in a grid, with each square in the grid referred to as a pixel or sample. The intensity of the image at each pixel is translated into a numerical value which is stored in the array. If the pixel has one of the primary colors (i.e., R, G, B) to be sampled with 8 bits resolution, then there are approximately 2.5 Mbits of data per frame (307K*8), which requires approximately 307 Kbytes of memory storage capacity to store the sampled image. Furthermore, if a digital still camera (DSC) is required to capture 1 frame/sec, a data transmission rate or bandwidth of approximately 2.5 Mbits/sec is needed for a color image. Depending on the number of pixels per frame and the image processing and compression circuits, the memory storage and bandwidth requirements can be much higher than 307 Kbytes and 2.5 Mbits/sec, respectively.
In particular, it may be desirable for digital still cameras to capture images at a high rate, i.e., in “burst mode”, for situations where frames are shot in rapid succession, such as during sporting events. For example, if a burst mode requires that 5 frames/sec be captured for high quality images of 2.0 Mbits/frame, then a minimum transmission rate or bandwidth of approximately 80 Mbits/sec or 80 MHz is needed. Further, if this burst mode is sustained for 5 seconds, then the storage capacity of the memory would need to be approximately 400 Mbits. Such large memories and bandwidths are either impractical or may require high capacity memory devices and very high speed circuit elements with parallel processing which can make the digital processing system complicated and very costly. Such a system can also require large amounts of power and board space.
Thus, the large memory storage and channel capacity requirements for digital image transmission and storage make it desirable to reduce the amount of digital data from A/D converter 120. By reducing the amount of data, the transmission bandwidth and/or the memory storage requirements are reduced. A well-known technique is to utilize image processing and compression circuits 130 to reduce the amount of digital data while storing enough data in memory to maintain a desired quality or resolution of the image. Image processing and compression circuits 130 can use common techniques, such as JPEG or run-length coding, to take advantage of the fact that certain pixel values tend to be highly redundant, such as with neighboring individual pixels or neighboring frames. Image processing and compression techniques, e.g., having compression ratios ranging from 4:1 to 20:1, can significantly reduce the number of bits required to represent images by removing these redundancies while still maintaining an acceptable quality of the reconstructed image.
After image processing and compression, the reduced digital data can be stored in a digital memory device 140, such as a memory card having a Flash EEPROM. Read and interface circuits 150 then access the desired image data from the memory device 140 and transmit the data to desired digital destinations, such as an LCD display 155, an external memory card 160, or a hard disk drive (HDD) in a PC (not shown). For example, with a digital still camera, elements 110 to 155 are typically embedded within the body of the digital camera so that digital memory 160 can be a memory card which is removable from the camera and available for subsequent processing of the stored image data. Data in the removed memory card can also be downloaded to a PC HDD or a kiosk for printing, used for later retrieval, or transmission through the Internet.
Thus, with image compression, smaller memories and lower transmission rates are possible. However, image processing and compression also require substantial amounts of time for performing complex and lengthy computations, which can adversely increase the minimum time delay between successive picture frames and/or burst rates. A high burst rate, which allows pictures to be taken in rapid succession, is an important consideration for high-speed professional and high-end, as well as typical, consumer cameras. At present, the duration that a digital camera can sustain for a high burst rate capture is very limited, e.g., typically two seconds or more between successive frames, due to the time required for image processing and compression. Furthermore, cameras capable of high speed image capture are typically high-end professional cameras with electronic drives and a large amount of memory, which can be large and expensive and make such systems undesirable for the typical consumer market.
Accordingly, a simple, low-cost and high quality digital imaging system for high speed image capture is desired which overcomes the problems discussed above with the conventional systems.